On the reputation calculation front, opinions and ratings for a community's member must be aggregated by the protocol from other reputable members' ratings, whose reputations also must be calculated. Chickens and eggs.

Consistency with others' opinions may be seen as a marker of conformity to a group, and a new datum from that member may be treated as extrapolated from the conformed group. An analyzed group's internal variability should also inversely influence the weight of an extrapolated opinion.

These are definitely not necessary for a simple "what are my following listing today?" Marketplace, but would instead be incredibly useful for wide markets.